Brandon123You'll probably get a lot of replies recommending the 305s because they are much more popular, but actually having heard both I give the thumbs up to the MKIIIs by a landslide. I have not heard the MKIIs so can't help there. The MKIIIs are absolutely fantastic for the price; with better marketing and distribution I see no reason why they wouldn't be the standard go-to recommendation for mid-fi active speakers.
BrandonlimIIRC bass response was stronger on the MKIIIs, but I have a Martin Logan sub hooked up anyway as I enjoy bass. The MKIIIs are not a flat response monitor like many other active speakers -- treble and bass are slightly pushed to make more a more enjoyable listening experience, as opposed to clinical sound mixing/mastering. With all that said I find the mids plenty rich and enjoyable -- these speakers use 60W per channel (the JBL does 40 by way of comparison) and there is an active crossover so you get a good meaty sound.
edit: 6moons has an incredibly in-depth and balanced review of the MKIIIs that I would recommend reading before making a decision.